1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotographic type or electrostatic recording type. More particularly, the present invention relates to an image forming apparatus configured to form an image on a recording medium, the apparatus including an image bearing member for bearing an electrostatic latent image on its surface, such as an electrophotographic photosensitive member or an electrostatic recording dielectric, and a developing apparatus for developing the electrostatic latent image with a developer (hereinafter referred to also as a “toner”).
2. Description of the Related Art
Japanese Patent Laid-Open No. 4-234777 discloses a method of detecting the amount of a toner remaining in a developing apparatus. The developing apparatus described in Japanese Patent Laid-Open No. 4-234777 includes a toner bearing member for supplying the toner to the toner bearing member and developing an electrostatic latent image, and a toner supply member held in contact with the toner bearing member and supplying the toner to the toner bearing member. In that developing apparatus, the amount of the remaining toner is detected by applying an alternating voltage to the toner bearing member from a development bias power supply, and by detecting a voltage induced in a core metal of the toner supply member. In other words, the electrostatic capacitance between the toner bearing member and the core metal differs depending on states of the remaining toner, i.e., a state where the toner is sufficiently present in the developing apparatus in such an amount as filling a space between the toner bearing member and the core metal, and a state where the toner is consumed with repeated image formations and the amount of the toner between the toner bearing member and the core metal is reduced. The above-described method enables the amount of the remaining toner to be detected without needing an extra space.
In the above-described method of detecting the amount of the remaining toner, however, a variation may occur in the detected electrostatic capacitance when the toner density within the developing apparatus is changed in spite of the toner being not consumed in the developing apparatus. Usually, during the image formation, the toner in the developing apparatus is sufficiently stirred and circulated with, for example, rotation of the toner bearing member and rotation of the toner supply member. On the other hand, when the developing apparatus is left unused for a long time after the end of the image formation, the toner in the developing apparatus is gradually compacted with a higher density at a position vertically approaching the bottom of a toner container downwards. Consequently, the toner density between the toner bearing member and the core metal of the toner supply member may be changed, thereby causing a variation in value of the detected electrostatic capacitance.
To solve the problem mentioned above, Japanese Patent Laid-Open No. 2010-026497 proposes a method of measuring the electrostatic capacitance between the toner bearing member and the toner supply member at a position differing from the position where the developing operation is carried out.
One example of an image forming apparatus employing the above-mentioned method will be described below with reference to FIG. 13. FIG. 13 illustrates a rotary-type image forming apparatus utilizing the technique described in Japanese Patent Laid-Open No. 2010-026497. Specifically, FIG. 13 illustrates the relative positional relationship among a photosensitive drum 1 serving as an image bearing member, four developing apparatuses 5a to 5d, and a rotary 50 serving as a developing-apparatus holder that holds the four developing apparatuses. Toners T in yellow, magenta, cyan, and black are filled respectively in the four developing apparatuses 5a to 5d. Each of the developing apparatuses 5a to 5d includes a developing roller 52 (one of 52a to 52d) serving as a toner bearing member, and an applying roller 53 (one of 53a to 53d) serving as an electroconductive support. The photosensitive drum 1 is rotatable by a controller (not shown) in the direction denoted by an arrow R1 in FIG. 13. Also, the rotary 50 is rotatable about a rotation shaft 51 in the direction denoted by an arrow R2 in FIG. 13. A rotation phase of the rotary 50 and postures of the developing apparatuses (5a to 5d) are now described. The posture of the developing apparatus at a development position where the developing roller 52 and the photosensitive drum 1 are contacted with each other is denoted by C. The posture of the developing apparatus at a position where the rotary 50 has rotated through 90° from the position corresponding to the posture C is denoted by F. The posture of the developing apparatus at a position (hereinafter referred to as an “electrostatic-capacitance detection position”) where the rotary 50 has further rotated through 90° from the position corresponding to the posture F is denoted by E. The posture of the developing apparatus at a position where the rotary 50 has further rotated through 90° from the position corresponding to the posture E is denoted by G. As seen from FIG. 13, the toners T are present around the applying rollers 53a, 53b and 53d when the developing apparatuses are in the postures C, F and G.
In the related-art image forming apparatus described above, the electrostatic capacitance between the developing roller 52 and the applying roller 53 is detected in the electrostatic-capacitance detection position (posture E) where the toner having deposited in a nip between the developing roller 52 and the applying roller 53 during a time in which the developing apparatus 5 has been held at the development position (posture C) is fallen in the gravitational direction. Therefore, the electrostatic capacitance between the developing roller 52 and the applying roller 53 can be measured without undergoing a variation in the detected electrostatic capacitance, which may occur due to change of the toner density between the developing roller 52 and the applying roller 53.
In the image forming apparatus disclosed in Japanese Patent Laid-Open No. 2010-026497, because the detection of electrostatic capacitance is performed at the electrostatic-capacitance detection position, the developing operation of the developing apparatus at the development position can be performed, when required, at the same time as the detection of the electrostatic capacitance. For example, when a full-color image forming operation is carried out in the image forming apparatus of FIG. 13, the developing operation for yellow (developing apparatus 5a) and the detection of the electrostatic capacitance for cyan (developing apparatus 5c) can be performed at the same time. Similarly, the developing operation and the detection of the electrostatic capacitance can be performed respectively at the same time for magenta (developing apparatus 5b) and black (developing apparatus 5d), for cyan (developing apparatus 5c) and yellow (developing apparatus 5a), and for black (developing apparatus 5d) and magenta (developing apparatus 5b).
However, the developing operation and the detection of the electrostatic capacitance are not performed at the same time in some cases. In one of those cases, for example, monochromatic image formation in black is continuously repeated. During the monochromatic image formation, the developing apparatus 5d performs the developing operation at the development position, but the amount of the remaining toner cannot be detected at the development position. In order to detect the amount of the toner remaining in the developing apparatus 5d, therefore, the rotary 50 needs to be rotated through 180° and moved to the electrostatic-capacitance detection position at a predetermined timing.
At the same time as when the developing apparatus 5d is moved to the electrostatic-capacitance detection position, the developing apparatus 5b for magenta is moved to the development position and is brought into contact with the photosensitive drum 1 to be ready for the developing operation. In the monochromatic image formation, however, because the developing apparatus 5b for magenta is not required to perform the developing operation, the developing apparatus 5b is stopped at the development position in the state contacting with the photosensitive drum 1 to wait for the end of the detection of the electrostatic capacitance in the developing apparatus 5d for black.
After the end of the detection of the electrostatic capacitance in the developing apparatus 5d for black, the controller resumes the driving of the rotary 50 to be ready for the next image forming operation. At that time, however, the toner on the developing roller 52b for magenta is moved onto the photosensitive drum 1 in some cases.
Such a phenomenon may occur due to various causes, for example, that the potential of the photosensitive drum 1 is attenuated during the detection of the electrostatic capacitance in the developing apparatus 5d for black, and that mechanical shocks are generated upon restarting of the rotary 50.
If the toner is moved onto the photosensitive drum 1 at the undesired timing as described above, the moved toner is further moved from the photosensitive drum 1 onto a transfer material, e.g., a sheet of paper, thus generating an image failure in the form of a streak or a stripe. From the ecological point of view that has particularly become valued in recent years, useless consumption of the toner is also undesired.